refactor(tools): group the sixty scripts by what they talk to

One directory with sixty scripts stopped being navigable, and the grouping is also the sandbox boundary:

  Tools/unreal/    commandlets and the two map builders, with ue_common.py and ue_placement.py as the
                   shared imports (they need the engine)
  Tools/blender/   builders, exporters, the FBX preset, the MPFB character pipeline, blender_screenshots
                   (they need the Blender Flatpak)
  Tools/review/    the visual loop: render_screenshots.sh, judge_screenshot.py, hero_compare.py,
                   build_comparison_md.py, e2e_asset_check.py, hero_iterate.sh, visual_loop.py,
                   visual_targets.json
  Tools/gen/       image and texture generation: the ComfyUI driver, deterministic texture writers,
                   reference fetching
  Tools/input/     OS-level input injection
  root             _common.py, check-lfs.sh, addon_compat_probe.py, sibling_asset_audit.py, tests/

Every moved script now resolves the repository root as ../.. and puts the tools root plus its siblings on
sys.path, so 'import ue_common' still works from unreal/. The test suite bootstrap does the same for the five
folders. render_screenshots.sh takes the folder as a third argument to run_cmdlet, and the e2e driver,
hero_iterate.sh and the comparison generator were repointed.

51 README rows now link to their new location, the README gains a Layout section naming each folder's
sandbox, and the architecture notes and Blender README follow the moves.

Verified: 108 tests pass, prettier is clean, judge_screenshot.py and hero_compare.py run from their new
homes, and the e2e run was aborted mid-way (it had been rebuilding an empty DemoMap for 15 minutes) and is
being rerun against the new layout.

Tools/unreal/ue_character_hair_import.py

@@ -0,0 +1,70 @@
+# Import the hair cards as their own skeletal mesh (0.2.2).
+#
+#   blender -b Art/blender/character/character-2026-11-07.blend \
+#       --python Tools/blender/character_hair_build.py -- --out /tmp/gigline/character-hair.fbx
+#   <engine>/Engine/Binaries/Linux/UnrealEditor-Cmd <uproject> -run=pythonscript \
+#     -script=<repo>/Tools/unreal/ue_character_hair_import.py -nullrhi -unattended -nopause -stdout
+#
+# The hair is a separate asset on purpose: Unreal collapses the second material slot
+# when hair is joined into the body mesh, so a card system has to be its own object
+# (which is what the character-artist skill asks for anyway).
+#
+# Result file: /tmp/gigline/character-hair-import.txt
+
+import unreal
+
+SRC = "/tmp/gigline/character-hair.fbx"
+DEST = "/Game/Test/Character"
+NAME = "CharacterHair"
+RESULT = "/tmp/gigline/character-hair-import.txt"
+lines = []
+
+
+def log(msg):
+    lines.append(str(msg))
+    unreal.log(str(msg))
+
+
+def main():
+    task = unreal.AssetImportTask()
+    task.filename = SRC
+    task.destination_path = DEST
+    task.destination_name = NAME
+    task.automated = True
+    task.save = True
+    task.replace_existing = True
+
+    ui = unreal.FbxImportUI()
+    ui.import_mesh = True
+    ui.import_as_skeletal = True
+    ui.import_animations = False
+    ui.import_materials = True
+    ui.import_textures = False
+    ui.skeletal_mesh_import_data.import_uniform_scale = 1.0
+    task.options = ui
+    unreal.AssetToolsHelpers.get_asset_tools().import_asset_tasks([task])
+
+    mesh = unreal.EditorAssetLibrary.load_asset(f"{DEST}/{NAME}")
+    if mesh is None:
+        log(f"MISSING {DEST}/{NAME}")
+        lines.append("VERDICT: FAIL")
+        return
+    mats = mesh.get_editor_property("materials")
+    log(f"{NAME}: {len(mats)} material slot(s)")
+    for i, m in enumerate(mats):
+        face = m.get_editor_property("material_interface")
+        log(f"  slot {i}: {face.get_name() if face else 'None'}")
+    lines.append("VERDICT: PASS")
+
+
+try:
+    main()
+except Exception as exc:  # noqa: BLE001
+    import traceback
+    log(f"FAILED: {type(exc).__name__}: {exc}")
+    log(traceback.format_exc())
+    lines.append("VERDICT: FAIL")
+
+unreal.EditorLoadingAndSavingUtils.save_dirty_packages(True, True)
+with open(RESULT, "w") as f:
+    f.write("\n".join(lines) + "\n")